Mileage is subjective. Guidelines are just that a guide. Back in the REAL old days they had the Mobile Economy Run and each year every manufactured car was driven in the real world over a substantial course with monitor guys on board to make sure there was no cheating. For example there was no neutral coasting on downgrades. But the stories got out how they cheated any how.
WW I ace Eddie Rickenbacker emphasised how mileage was an individual achievement. He'd say he would drive a car someone got great mileage out of and he couldn't do it.
After the CAFE limits were instituted they resulted in some hokey figuring completely unlike real world driving. It's akin to the times when the insurance companies got tough about HP and so manufacturers began reading HP at the end of the transmission instead of at the flywheel for a lower reading.
It's the same with any government mandated fuel economy projection. Now they supposedly got a new new/better way of figuring it. Yeah. They can make it come out the way they want depending on subtle things.
About all we can tell from the BS on the stickers is if one car says 21/33 mileage and another says 23/31 that the 1st car will get somewhat better mileage....though yours may vary